In recent years, aesthetic dental treatment has been drawing attention in the dental field, and a restorative procedure in which a dental material having high aesthetics for a color tone that is similar to that of natural teeth has been put into practice. In such circumstances, a dental curable composition called a “composite resin” composed of a polymerizable monomer, a filler, a polymerization initiator, and so forth is clinically most heavily used as a material for restoring a chipped portion of a tooth from the viewpoint of operability, mechanical properties, and aesthetics. Clinical application of the material is very satisfactory from the viewpoint of aesthetics because the material enables aesthetic restoration in which the color tone of the restored portion is so similar to that of the tooth substance that the restored portion is indistinguishable from the natural teeth after filling the chipped portion. Meanwhile, the natural teeth have fluorescence properties to generate fluorescence when irradiated with an ultraviolet ray and to turn pale under black light. Unlike the natural teeth, however, the dental curable composition originally does not have fluorescence properties. Therefore, when an object restored using the composition is seen under an ultraviolet ray or black light, the object may look darker than the surrounding teeth, which may degrade the aesthetic quality. Therefore, dental curable compositions containing various fluorescent substances for the purpose of exhibiting fluorescence properties that are similar to those of the tooth substance are proposed. For example, JP2012-505889A discloses a dental curable composition containing a fluorescent organic dye. JP2011-184402A discloses a composition containing a fluorescent substance formed from alumina particulates and a compound having a benzophenone skeleton. JP02-233605A discloses a composition containing 2,5-dihydroxy diethyl terephthalate. JP2009-510120A discloses a dental composition containing 7-diethylamino-4-methylcoumarin. JP2005-41825A discloses a dental material containing a fluorescent glass filler containing a rare-earth oxide. Such disclosed technologies have enabled dental curable compositions to look similar to the natural teeth when irradiated with light in an ultraviolet or near-ultraviolet range that is shorter in wavelength than visible light.
In recent years, in addition, explorations and diagnoses have been rendered important in dental treatment, and it has become necessary to check the clinical status and information on a portion to which a dental material is applied. In particular, dental compositions such as a fissure sealant applied to the tooth substance and an orthodontic adhesive remaining on the tooth surface after removal of a correcting bracket are required to be visually recognizable in order to grasp the status of the procedure or decide the next policy of the procedure by identifying the presence of such dental compositions. The dental compositions containing various fluorescent substances proposed earlier are characterized in that they look in the same way as the tooth substance both with the eyes and under irradiation with an ultraviolet ray or black light, and are found to be unsatisfactory from the viewpoint of visual recognizability to identify the presence of the dental compositions. Conversely, the visual recognizability of the dental compositions can be improved by having a fluorescent material having fluorescent chromogenic properties, which are completely different from those of the tooth substance, contained in the dental compositions. However, it is clearly not preferable in terms of biological safety to irradiate an oral cavity with an ultraviolet ray or black light in the course of an exploration or a diagnosis. Further, while it is possible to have a pigment with a color tone that is completely different from the color tone of the tooth substance contained in the dental compositions for the purpose of enhancing the visual recognizability with the eyes, this may result in degradation in aesthetics with the eyes.
There is further proposed a dental composition that can be visually recognized by irradiation with visible light or a visible ray by having a fluorescent substance that generates fluorescence by irradiation with visible light or a visible ray contained in the dental composition as in the invention taught in JP2005-41825A. In the dental composition taught in [0027], [0028], [0033] to [0035], and so forth of JP2005-41825A, a glass filler (fluorescent substance) containing Eu2O as a rare-earth oxide that generates fluorescence when irradiated with visible light or a visible ray is mixed in a visible light-curable composite resin. In the dental composition, however, the glass filler (fluorescent substance) containing at least 5 wt % of an oxide of Eu itself is mixed in an amount corresponding to a weight ratio of 3:1 based on the composite resin, and the content of the rare-earth oxide as the fluorescent substance to generate fluorescence may be too large. Because a large amount of the fluorescent substance is present in the dental composition, the dental composition may be colored by the fluorescent substance, which may degrade the aesthetics under natural light. If the content of the fluorescent substance is increased, the mechanical strength including the bending strength of the dental composition after being cured may be reduced, depending on the type of the fluorescent substance.